USB is a standard interface for connections or communications of computer systems with its peripherals, which is divided into low-speed devices and full-speed devices. FIG. 1 illustrates a connection manner of USB low-speed device, of which a host computer 10 and a USB low-speed device controller chip 12 are connected together by a cable 14 with connections of the respective D+ ports 16 and 20, and D− ports 18 and 22. There are two pull-down resistors Rpd of 15 KΩ connected from the D+ and D− ports 16 and 18 to ground, respectively, at the host computer 10 side as well as a pull-up resistor Rpu of 1.5 KΩ connected between the D− signal port 22 and a supply voltage of 3.3 V at the USB low-speed device controller chip 12 side. On the other hand, there is a difference between the USB full-speed device and the low-speed device in that the pull-up resistor Rpu for the USB full-speed device is connected to the D+ signal port, instead of the D− signal port as in the manner for the USB low-speed device shown in FIG. 1.
The USB specification defines the pull-up resistor Rpu of 1.5 KΩ with a tolerance not larger than 5%. The widely used ¼ Watts carbon film resistor has a resistance tolerance thereof just meet 5%. However, the USB low-speed device controller chip 12 together with an external pull-up resistor Rpu results in a higher cost, and the controller chip 12 needs to be provided with one additional pin for connection of the external pull-up resistor Rpu and thus not only increasing the cost, but also increasing the possibility of being damaged by electro-static discharge (ESD). In addition to the cost down of the device manufacturer, the pin count on the chip package will be decreased by one and thereby is decreased the possibility of ESD damages if the pull-up resistor Rpu could be integrated within a single chip. Nevertheless, the polysilicon resistor formed by the normal semiconductor processes in combination with the temperature drift may have a tolerance approximately ranged from 10% to 20%, it is therefore difficult for the pull-up resistor Rpu to be built in a single chip to satisfy the USB specification requirement, unless special and thus expensive process is used.
In consideration of the definition for the pull-up resistor Rpu in the USB specification, it is realized that the main function of that resistor Rpu is to identify a connected device is a USB full-speed one or a low-speed one depending upon which signal between D+ and D− signals is pulled high (logic “1”) as the USB device is plugged in a system. In terms of this aspect, it is noted that it will not destroy the normal operation of a system as long as the voltage divided by the pull-up resistor Rpu of 1.5 KΩ and the pull-down resistor Rpd of 15 KΩ will not result in a misjudgment of high level by digital logic, even though the pull-up resistor Rpu has a larger tolerance range than the USB definition.
It is therefore desired a chip integrated with a USB pull-up resistor.